The present invention relates to a balance arm for flat weighing machines, in particular weighing machines for shops, having two pairs of knife-edges which are located on a rigid frame.
Weighing machines of high accuracy necessitate balance arms having the most complete alignment possible of the sharp edges of each pair of knife-edges, one pair of knife-edges usually forming the so-called main bearing or support bearing, whilst the sharp edges of another pair of knife-edges are load-carrying knife-edges and serve to introduce the force to be measured. In addition to the complete mutual alignment of the two sharp edges of one pair of knife-edges, it is also necessary that all sharp edges of the various pairs of knife-edges lie in a common plane.
This requirement is best satisfied, from the point of view of production engineering, by machining the sharp edges (or function lines) of the coarse knife-edges, or raw knife-edges, which have been attached to the balance arm, on a common grinding machine. By using the same grinding machine it is possible to ensure, by known techniques, that all sharp edges lie, parallel to each other, in a common plane. For example, the balance arm may be rotated through 180.degree. during the grinding operation. Inadequate alignment would cause increased friction, which would manifest itself as inadequate mobility. Inadequate parallelism and inadequate location in a common plane would result in excessive errors in the case of so-called corner loading.
The precision-grinding, in common, of the knife-edges, located on a common balance arm, necessitates a special configuration of the balance arm, this configuration guaranteeing that each sharp edge can be ground along its entire length. In the main, two configurations of such a type are being employed in practice to date. The first known configuration relates to the so-called S-head arm, in which the knife-edges are attached to S-shaped arms with the sharp edges turned towards each other, in such a manner that they can be ground along their entire length. S-head arms of this type are, for example, described in "Waagen, Bau und Verwendung" (Weighing Machines, Construction and Use), Erwin Hess, 1963, pages 40 to 43. Having regard to the need for stiffness, S-shaped balance arms have the disadvantage of a relatively large assembly height and permit only relatively large distances between the two pairs of knife-edges. In the other balance arm configuration which is used in practice, a balance arm in the form of a frame, made, for example, from flat material, is provided with conical bores at the sides, into which knife-edges which have previously been ready-ground (so-called "pear knife-edges", for example) are pressed, by means of a conical extension, these knife-edges projecting laterally in the manner of a cantilever. Such an arrangement of knife-edges is described and illustrated in the above-mentioned publication, pages 43 to 45. Although this configuration certainly permits a comparatively low assembly height and small distances between the pairs of knife-edges, the advantageous common grinding of all pairs of knife-edges attached to the balance arm, along the entire length of the sharp edges, is nevertheless impossible. The bores for the knife-edges of each pair of knife-edges must be in precise mutual alignment, and the sharp edge must coincide as accurately as possible with the axis of the conical extension of the knife-edge. Considerable precision is accordingly necessary in producing the bores and knife-edges, without at the same time attaining the quality of the S-head arm, with regard to weighing technology.
It is an object of the invention to remedy the disadvantages described, and to provide a balance arm which is of flat construction and which permits short distances between the pairs of knife-edges.